With an increase in demand for communication, optical networks using wavelength division multiplexing (WDM) have become widely prevalent. Wavelength division multiplexing is a technology for multiplexing a plurality of optical signals with different wavelengths and transmitting a multiplexed signal. Wavelength division multiplexing is capable of, for example, multiplexing optical signals with 40 wavelengths at a transmission rate of 40 Gbps and transmitting the optical signals as a multi-wavelength optical signal (hereinafter referred to as a “multiple optical signal”) at 1.6 Tbps.
For example, a reconfigurable optical add-drop multiplexer (ROADM) apparatus is known as an optical transmission apparatus using WDM. A ROADM apparatus is provided with an optical amplifier which amplifies a multiple optical signal input from a transmission path. A gain of the optical amplifier is controlled in order to compensate for a transmission loss in the transmission path and secure predetermined power of a multiple optical signal.
The gain of the optical amplifier is controlled by, for example, adjusting an attenuation of a variable optical attenuator (VOA) which is provided between two erbium-doped fiber amplifiers (EDFAs) which amplify a multiple optical signal. However, to expand a control range for the gain to cope with a wide variety of transmission loss, expansion of a variable range for the attenuation of the VOA is desired. The expansion degrades a noise figure (NF) of the optical amplifier.
For this reason, the optical amplifier is configured to select an appropriate one from among the plurality of EDFAs different in amplifying characteristic with an optical switch or the like. The range of control of the gain is expanded by adjusting the attenuation of the VOA.
The gain of the optical amplifier is desirably flattened to be steady regardless of wavelength so as not to cause a difference in power between light rays of individual wavelengths included in a multiple optical signal. However, if an EDFA to amplify a multiple optical signal is selected from among the plurality of EDFAs, as described above, an amplifying characteristic depends on the selected EDFA. This suffers from the problem of the difficulty in flattening a gain.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 5-218974 and
[Document 2] Japanese National Publication of International Patent Application No. 11-509070.